1. Field of Invention
This invention relates generally to a method and device for optical sensor compensation to automatically adjust for sensor degradation, and an apparatus incorporating the same.
2. Description of Related Art
Fluid ejector systems, such as drop-on-demand liquid ink printers, have at least one fluid ejector from which droplets of fluid are ejected towards a receiving sheet. Scanning inkjet printers are equipped with fluid ejection heads containing fluid ink. The ink is applied to a sheet in an arrangement based on print data received from a computer, scanner or similar device. To control the delivery of the fluid to the sheet, fluid ejection heads are moved across the sheet to provide the fluid to the sheet, which is ejected as drops. These drops correspond to a liquid volume designated as pixels. Each pixel is related to a quantity needed to darken or cover a particular unit area.
An exemplary fluid ejector system is a thermal ink jet printer. FIG. 1 illustrates a partial schematic perspective view of an ink jet printhead cartridge 26 mounted on a carriage 23 supported by carriage rails 24. The printhead cartridge 26 in housing 25 contains ink for supply to a thermal ink jet printhead 20, which selectively expels droplets of ink under control of electrical signals received from a controller 27.
The carriage 23 is moved back and forth in the scanning directions 28 by a belt 29 attached thereto. The belt 29 is moved by a first rotatable pulley 17 and a second rotatable pulley 18, one of which is driven by a motor (unshown).
To control the movement and/or position of the carriage 23 along the carriage rails 24, the printer includes an encoder having an encoder strip 21 which includes a series of fiducial marks 19 in a pattern. The pattern is sensed by a sensor 22, such as a photodiode/light source, which may be attached to the printhead carriage 23. The sensor 22 transmits electrical signals representing the sensed fiducial marks 19 to the printer controller 27 to thereby measure actual printhead position as it reciprocates along carriage rails 24. Encoder 21 may be formed of a thin film of transparent plastic with opaque scaled lines to provide linear fence encoding. The encoder runs parallel with carriage movement along carriage rod 24. The encoder 21 may be provided under the encoder sensor 22 for displacement detection with a movement of the scanning carriage. Alternatively, rather than having sensor 22 contain both a light source and photodetector on the same side, one of them may be provided on the opposite side of encoder film 21 to sense light transmitted through transparent portions of the film.
As another example, a printer may utilize a rotary encoder having a code disc with a series of radially arranged fiducial marks 19 in a pattern. The pattern is sensed by a sensor, as described above for the linear encoder. The sensor transmits electrical signals representing the sensed fiducial marks 19 to the printer controller. Thus, a rotary encoder and sensor as described may also be used by a printer for optical measurements of lengths and angles.
Often times one or more other optical sensors are provided in near proximity to the printhead 20. FIG. 2 shows such an arrangement in which ink drops 14 are ejected from an ink jet nozzle 11 of printhead 20. The two optical sensors 12 and 13 are shown located in the same scanning carriage 23 with the thermal ink jet printhead 20. These sensors 12 and 13 typically are disposed toward the imaging platen 15 where a sheet of paper is typically positioned. One of the sensors 12 senses the page position of the paper in order to locate the image on the paper. Another of the sensors 13 typically senses the location of printing on the paper in order to align the color and black printheads to each other.
In FIG. 3, a light emitting device (LED) 5 forming part of an optical sensor 30 is shown with a pull-up resistor 31. A phototransistor 6 is shown as a common-emitter amplifier to detect the emitted light from LED 5. The output Vout is created by connecting a resistor 33 between the voltage supply Vcc and the collector terminal of the phototransistor 6. Depending on the actual configuration of the optical sensor 30, the phototransistor 6 may detect a reflected light or a transmitted light. When optical sensor 30 is used as a page position sensor or a color alignment sensor, such as sensors 12 and 13, light reflected from a paper is detected by the optical detector (phototransistor) 6. For encoders having a combination of sensor 22 and encoder 21, such as a linear fence encoder or a rotary encoder, light may be reflected from the encoder as shown in FIG. 1, or alternatively, light transmitted from the optical emitter 5 through the encoder may be detected by the optical detector 6.
A recurring problem in ink jet printing or xerographic copying is the contamination of optical sensors, particularly those in the paper path. For example, Thermal Inkjet (TIJ) printing ejects small drops of ink onto paper. Referring to FIG. 2, during the ejection of the droplets 14 from the printhead 20, when the droplets spray from the printhead 20, tiny droplets called satellites may form a “misting.” Also, after the droplet 14 hits the target medium, it can sometimes “splash” from the surface, creating tiny airborne droplets. These droplets and satellites are very small, on the order of a picoliter or less. They become airborne in the high velocity air currents from the printhead 20 traversing during its printing swath.
Particularly, during the life of the ink jet printer, these airborne particles together may become contaminants that coat the optical elements over time, causing eventual malfunction or failure, requiring maintenance or replacement. Other contaminants, such as dust, also accumulate over time within the printer and coat the optical elements.